Archive for March, 2008

Surprisingly, quantum physicists cannot agree. Some say the measurement ends when you register a result on a piece of classical equipment such as a photomultiplier. Others says the measurement ends when the information in the quantum system has irreversibly leaked into the environment. There are still more who believe in the manyworlds interpretation of quantum mechanics and say a quantum measurement never ends but exists ad infinitum in several parallel universes.

This may sound like an ineffectual academic scrap but it actually has hugely important consequences for the quantum property of entanglement.

Entanglement is the state in which two physically separated particles share the same quantum existence, so that a measurement on one instantaneously affects the other. Yep, that’s instantaneously. It’s what Einstein described as “spooky action at distance”.

For some years, physicists have been measuring this “spooky action at a distance” in tests known as Bell experiments.

These tests depend crucially on the measurement ending quickly. Because if it were to drag on, the particles might be able to communicate at light speed by some currently unknown mechanism.

But because nobody has actually determined when a measurement ends, all the experiments to date are potentially open to this loophole.

Perhaps there is no spooky action at a distance after all, just long quantum measurements during which the particles communicate at the speed of light in some quite ordinary way.

Now Nicolas Gisin and colleagues at the University of Geneva have closed this loophole using the ideas of the Oxford theorist Roger Penrose. A few years ago, he suggested that the end of a quantum measurement is realted to the gravitational energy of the mass distribution of the resulting quantum superposition. In other words, the measurement ends when a massive object receives a decent kick.

So Gisin and buddies set up a Bell experiment which involved sending entangled photons in each direction from the midpoint of an 18 km fibre. At the ends of the fibre were piezoelectric actuators attached to small but massive mirrors. When the photons hit, they triggered the actuators causing the mirrors to move and deflect a beam of light.

The experiment was carefully set up so that the mirrors were heavy enough to please Penrose and far enough apart that no light speed signal could travel between them in the time it took for a pair of entangled photons to “kick” them.

The result? Gisin’s team confirmed that “spooky action at a distance” still governs the behaviour of the entangled photons.

If you believe Penrose, this is the first experiment to ever prove “spooky action at a distance”. Impressive, huh?

More interesting, is the idea of gravity and quantum mechanics coming under the microscope in the same experiment for the first time.

It won’t be the last. There are plenty of other mysteries about gravity that quantum mechanics can probe. It’s about time physicists bit the bullet and started testing them.

A vipers nest on the arXiv today from two groups covering the question of whether cosmic rays can trigger cloud formation and may therefore be a significant player in the global warming debate.

The thinking goes like this: cosmic rays ionise the atmosphere, triggering the formation of aerosols which in turn nucleate cloud cover. The number of cosmic rays hitting Earth is determined by the Sun’s magnetic field which follows an 11 year cycle. Cloud cover should therefore follow a similar 11-year cycle (and any other changes in the solar magnetic field over longer time scales).

The evidence is more complicated.

On the one hand, we have Vitaliy Rusov from the National Polytechnic University in Odessa, Ukraine, and colleagues who argue that the Earth’s climate is governed by only two factors: the amount of incoming solar radiation and the cosmic ray flux which determines cloud cover and therefore effects the amount of radiation that is reflected and absorbed. In two papers examining the theory and data, Rusov and co argue that carbon dioxide levels play no part in determining our climate.

On the other hand, we have Terry Sloan from the University of Lancaster and a pal who have studied the evidence that links cloud cover to cosmic ray flux. They point out that in some places the cloud cover data appears to correlate with cosmic rays, in others it anticorrelates. And since cosmic ray flux changes with latitute, so should cloud cover although there is no data to support this. They conclude that if cosmic rays do influence cloud cover, then they can be responsible for no more than 23 per cent of globally averaged cloud cover changes during the 11-year solar cycle.

I think I can see where the wind is blowing on this one. But it needs some heavyduty contemplatin’ by some serious climatologists and atmospheric physicists.

The International Panel on Climate Change has yet to pronounce on this effect and its influence on climate. But it needs to move quickly to determine whether it should be a significant part of the climate debate or a red herring.

New types of stars aren’t found very often but last year, Patrick Dufour and pals discovered several white dwarfs with carbon atmospheres. Before then white dwarfs were thought to come in two flavours: with atmopsheres dominated by either hydrogen or helium. Astronomers suddenly had a new toy to play with.

Dufour found nine examples of his carbon dwarfs in the data regurgitated by the Sloan Digital Sky Survey and more are likely to be found as the skies continue to be searched.

So what are white dwarfs with carbon atmospheres like? Dufour and some theorists have been working out the details and today publish their results. Carbon dwarfs should be great throbbing balls of fire. Yep, pulsating stars.

They’ve had a bit of luck here. Turns out that Gilles Fontaine at the University of Montreal produced a theoretical study of carbon-based white dwarfs for his phd thesis 35 years ago, long before they were even discovered.

A quick look through his notes has revealed that these stars should pulsate as carbon is cycled through the atmosphere by convection (although the exact details depend on the amount of carbon in the stellar atmosphere).

Now the hunt is on to find more of these rare white dwars and to measure the amount of light they produce. “We are eagerly awaiting the results,” say Fontaine and Dufour.

Having eagerly waited all of one day, Dufour and a few buddies have announced the discovery of a pulsating carbon-rich white dwarf.

Congratualtions to them but there is a potential fly in the ointment: what looks like a pulsating dwarf could actually be a binary system of two white dwarfs. Dufour is unfazed. He points out that the characteristics of the system are unique so either way, they’ve found a new class of something or other.

He finishes with this: “We will continue the search for other objects of this exciting and enigmatic class.”

This is the idea behind quantum imaging: create an entangled pair of photons and send one towards the object you want to image and hang on to the other.

But then what? For some time, physcists have been whisperin’ about the extraordinary potential of this technique. Some imagine that it might be possible to create images of objects that cannot otherwise be seen, objects inside black boxes, for example, or black holes.

The thinking is that the photon you hold in your hand can somehow tell you something about the object it’s entangled cousin has hit. So you can create an image of an object without ever seeing its reflection.

But pin physicists down about what they mean and they start a-mumblin’ and a-dribblin’ incoherently. Despite rumours from places like Boston University where various bods are testing the idea in the bowels of the physics department, nobody has ever provided experimental evidence that quantum illumination is anything but a hatful of hot air.

So if ever a field needed an injection of common sense, this is it. Step forward quantum theorist and all round bright spark Seth Lloyd from MIT. He’s taken the thinkin’ and given it a thorough shakin’ by the scruff of its neck.

Lloyd doesn’t give any credence to the ideas of reflection-free imaging but he’s found something almost as good. Lloyd has calculated that illuminating an object with entangled photons can reduce increase the signal to noise ratio of the reflected signal by a factor of 2^e, where e is the number of bits of entanglement. That’s an exponential improvement.

What’s more, the improvement occurs even if the entanglement is completely destroyed during the process of reflection. So quantum illumination could help image anything that is currently hard to distinguish because of noise.

That’s impressive but Lloyd’s ideas raise quite a few questions, such as how to perform the required entanglement measurement on the returning photon. That’s for the experimentalists to sort out although there’s no easy and obvious answer.

So although a clever piece of work, it could be a while before we’re posing for quantum snapshots from Kodak.

Quantum physicists have been sending qubits through the atmosphere encoded in individual photons for years now. The work is the foundation of a new type of quantum communication that is perfectly secure from eavesdropping.

But there are challenges in setting up a global system of quantum communication. Not least is the problem of decoherence, in which noise destroys the quantum nature of the information as it travels though the atmosphere. This has limited the distance record for this kind of transmission to 144km (although longer distances are possible through optical fibres).

The obvious way around this is to send the signals through space via a satellite. When sent straight up, the photons need only travel through 8 kilometres of atmosphere and so are much less likely to decohere.
On Friday, Anton Zeilinger’s group in Vienna announced that they had taken the first step in this direction by bouncing single photons off an orbiting satellite soome 1400km above the Earth.

The team used a 1.5 metre telescope called the Matera Laser Ranging Observatory in Italy to bounce single photons off the Ajisai geodetic satellite, an orbiting disco ball that is used for laser ranging measurements.

Quantum communication with entangled photons can only be done by sending and detecting them one at a time so the experiment is a crucial step in making space-based quantum communication possible.

However, the team also tried bouncing photons off several other disco balls such as Lageos II, without success.

But give them their due. The experiment proves that it is possible to use existing laser ranging equipment to send and receive single photons from orbiting satellites.

“Our findings strongly underline the feasibility of Space-to-Earth quantum communication with available technology,” says the team.

Of course, this isn’t a demonstration of quantum communication itself in space. That will require an orbiting source of entangled photons.

So all they need now is somebody to build and launch a satellite that can produce and transmit entangled photons. Any takers?

Ref: arxiv.org/abs/0803.1871: Experimental Verification of the Feasibility of a Quantum
Channel between Space and Earth

Jorge Horvath from the Universidade de Sao Paulo in Brazil seems to think so and believes that our current search for near Earth asteroids may uncover them.

Here is his thinkin’. About 20 years ago, a number of physicists investigated the possibility that a quark gluon plasma–a state of matter that should only have existed in the earliest instants of the universe–could become frozen and preserved. But how long would it survive?

Nobody knows how quickly this stuff boils away but plenty of estimates suggest there ought to be nuggets of this stuff floating round the universe today. Many of these nugggets should be about as massive as an asteroid but obviously much smaller because of their huge density.

Horvath has calculated that if these quark nuggets were captured by the Sun, there should be more than 10 million of them in orbit right now, adding several times the mass of the Earth.

So how to find them. Horvath says they should be perfect reflectors of optical light and so although speck-like in size, should look about as bright an ordinary asteroid (ie not very bright). But being perfect reflectors, the light should show the characteristic spectra of the Sun rather than an asteorid. They could also be identified by their vanishing occultation times.

All this, of course, assumes that the nuggets are clean and not surrounded by ordinary matter. That seems unlikely to me.

My guess is that if these things are out there, they’re gonna be covered with all kinds of muck. Does that mean that primordial quark nuggets could seed the formation of objects like comets and planets? Now there’s a thought.

Look at the finishers in a typical marathon and a simple pattern immediately emerges.

After the race winner, there is a trickle of fast finishers that gradually turns into a steady flow as the finish time approaches 3 hours. The main pack arrives in the range of 3–6 hours, with a decreasing stream of progressively slower stragglers.

It is easy to imagine that this distribution would be pretty smooth, given the number of runners in these events. Sanjib Sabhapandit and colleagues at Universite Paris-Sud in France thought so too until they looked at data from the entire field of male runners at races in Boston, Chicago, New York between 2000 and 2007.

It turns out that the distribution of finish times has some interesting characterstics. For a start, there are spikes just before the 3 and 4 hour marks as runners attempt to beat specific milestones. In the Chicago marathon, where the course is flat and relatively easy to pace, there are secondary peaks just before the times of 3:10 and 3:20.

“The existence of such peaks suggests that the distribution of finish times in this range does not reflect a performance limit, but rather, the surmounting of a psychological barrier,” say Sabhapandit and buddies.

Another feature is crowding at the front of the pack among runners who finish with a time less than 2:15. These are the elite runners who enjoy considerably incentives to maintain their competitive edge, such as appearance money and access to the best support institutions.

But acheiving a slightly slower time of between 2:15 and 2:30 is still a significant feat that requires a considerable investment in time. This cannot be done without financial support and so there is a deficit of runners in this cagetory. This is what makes the front seem crowded.

Of course, all these features are easy to predict given agood knowledge of the way marathons work.

What’s curious is that Sabhapandit says a similar crowding effect can be seen in human age distributions where the longest-lived individuals survive longer than the distibution would imply. “There seems to be a self-selected sub-population of advantaged individuals who gain advantage both innately and perhaps because of external reinforcement,” they say.

These people are analogous to the elite runners who receive a financial incentive to perform well.

If Sabhapandit is correct, what factor performs the role of appearance money for people running the race of life?

In recent years, physicists have turned their penetrating gaze towards the structure of towns and cities. What they tend to do is measure the “connectedness” of a town by looking at how many roads each street is connected to. It turns out, that cities follow an 80/20 rule, that 80 percent of the streets have a below average connectedness while 20 per cent have an above average connectedness.

This is no surprise since the same kind of 80/20 pattern crops up with alarming regularity in all kinds of networks, particularly social ones. (The most famous is Pareto’s law which states that 80 per cent of the wealth is owned by 20 per cent of the people).

But so what? Pawing over maps and sweating over street names maybe a theoretical physicist’s idea of fun but nobody has actually proved that the 80/20 rule has any tangible effect on street use.

Now Bin Jiang at the Hong Kong Polytechnic University has come up with some actual data from a real town. He says that 80 percent of the traffic in a Swedish town called Gavle flows along 20 per cent of the streets. And 1 per cent of the most highly connected steets account for a phenomenal 20 per cent of the flow. What’s more, he says the flow is intimately linked to the topology of Gavle (a town of 70,00 people).

So there you have it. Although it seems only common sense to imagine that the most traffic flows along the best connected streets, we now have some evidence to prove it. Good, solid, unspectacular physics.
Ref: arxiv.org/abs/0802.1284: Street Hierarchies: A Minority of Streets Account for a Majority of Traffic Flow